The problem occurred in the following scenario:
1. VLR sent PAGING-REQUEST to the MME
2. MME sent S1-Paging to the UE
3. Paging failed
4. MME responded SERVICE-REQUEST to the VLR
5. VLR sent DOWNLINK-UNITDATA to the MME
6. Even though there is no S1 Context,
MME try to sent DownlinkNASTransport message to the UE.
7. So, the problem occurred.
I've changed the number 4 PAGING-REJECT instead of SERVICE-REQUEST.
321c8d4cf9